Passengers flying to the US out of YUL
clear customs at YUL
before boarding. The flight from ATL
will either be a 757 or a 767. If it is a 767, it will probably depart from either Concourse A or possibly Concourse T. An MD
-80 flight will likely arrive at either Concouse B or D. Needless to say a change of concourses will likely be required. If the flight is on time, 45 minutes should be plenty of time. If not, it could be scramble. The concoures at ATL
are long. The underground shuttle arrives and departs from the center of each concourse. If the arriving flight from YUL
arrives near the shuttle station in one concourse and the departing flight to LAS
departs near the other shuttle station, than it will be easy to connect (only a small hike). If one of the flights departs or arrives from the end of one of the concourses, then there will be a longer hike, and a longer connection time would be required.
On advantage to DL
's hub at ATL
is the frequency of flights. If for chance your parents missed their connecting flight to LAS
, they could probably be re-booked on the next flight (provided it is not full) and only be stuck for say two to three hours. All of this assumes of course that the flight to LAS
you want to book them on is not the last flight of the day.
I would check the on-time performance of the flight from YUL
...most travel search engines have these statistics. If it has a good on-time performance, I would say that 45 minutes should be enough time.